VARIETIES OF OYSTERS. 143 



SHELL round, oval, oblong, cylindrical, angular, or 

 even amorphous, compressed, and sometimes flattened, of 

 different degrees of thickness according to age, outside of 

 a dull appearance, although the inner layers are remark- 

 ably glossy and iridescent : sculpture, scaly and sometimes 

 prickly, minutely striate in a longitudinal direction, and 

 marked by irregular lines of growth : colour white, with 

 often a yellowish, pink, rose-red, or brown tint : margins 

 thin, rounded or wavy unless contracted by position, nearly 

 forming an obtuse angle behind : beak straight and very 

 small, not projecting beyond the dorsal margin : cartilage 

 short but strong, broad, semilunar, and fixed in a cavity 

 underneath the beak : hinge-line slightly curved : hinge-plate 

 thick and broad : orifice oval ; outer edge reflected : inside 

 silvery and iridescent, sometimes having a green tint, 

 furnished in the lower valve at the hinge-end with a thick- 

 ened ledge, to receive and support the cartilage : muscular 

 scar large, showing in the upper valve the impressions of 

 three inner portions of the muscle, which are nearly cir- 

 cular and disposed in a descending but irregular line from 

 the hinge, and in the lower valve only one similar impres- 

 sion, which is placed on the right hand of the observer : 

 plug cylindrical, thick, and longitudinally striate. Length 

 2'3, breadth 2*5. 



HABITAT : From low water-mark to 80 fathoms on 

 every part of our coasts, attached to shells, stones, sea- 

 weeds, and other substances. In a fossil state it occurs in 

 our newer tertiaries, as well as in the Coralline Crag, and 

 in the Italian pliocene deposits. It is likewise found in 

 the post-glacial beds of Bohuslan, Sweden, associated with 

 arctic shells. It is widely distributed in the European 

 seas, from Iceland to the vEgean Archipelago ; and its 



